The present invention relates to a method for fabricating photomasks used for the production of high-density integrated circuits such as LSIs, VLSIs, etc., and more particularly to a method for fabricating a photomask having a phase shift layer that is used for forming fine patterns with high accuracy.
Semiconductor integrated circuits such as ICs, LSIs and VLSIs are now fabricated by repeating the so-called lithographic process wherein a resist is coated on the substrate to be processed, like a silicone wafer, and exposed to a desired pattern through a stepper, etc., followed by development, etching, doping, CVD, etc.
A photomask used for such a lithographic process and called a reticle is now increasingly required to have much higher accuracy in association with the high performance and high integration of semiconductor integrated circuits. Referring to a typical LSI, i.e., a DRAM by way of example, a 5.times.reticle for an 1 megabit DRAM, i.e., a reticle of a size five times as large as that of an exposure pattern should be very small in terms of dimensional variation; the accuracy demanded is as small as 0.15 .mu.m even at the mean value .+-.3.sigma.(.sigma. is the standard deviation). Likewise, a dimensional accuracy of 0.1 to 0.15 .mu.m is demanded for five-fold reticles for 4 megabit DRAMs; a dimensional accuracy of 0.05 to 0.1 .mu.m for five-fold reticles for 16 megabit DRAMs; and a dimensional accuracy of 0.03 to 0.07 .mu.m for five-fold reticles for 64 megabit DRAMs.
In addition, the line widths of device patterns formed with the use of these reticles are now becoming finer; for instance, they must be 1.2 .mu.m for 1 megabit DRAMs, 0.8 .mu.m for 4 megabit DRAMs, 0.6 .mu.m for 16 megabit DRAMs, and 0.35 .mu.m for 64 megabit DRAMs. To meet such demands, various lithographic technologies are now under investigation.
In the case of the next generation device patterns of the 64 megabit DRAM class for instance, however, the use of stepper lithographic technologies using conventional reticles will place some limit on resolving the resist patterns. To exceed this limit, a phase shift reticle, designed on the basis of a new technological paradigm, has been proposed in the art, as set forth in JP-A-58-173744, JP-B-62-59296, etc. Phase shift lithography making use of this phase shift reticle provides improved to be resolution and contrast of a projected image to be transmitted by manipulation of the phase of light transmitting through the reticle.
Phase shift lithography will now be explained briefly with reference to FIGS. 2 and 3. FIG. 2 is a schematic of the principle of the phase shift process, and FIG. 3 is a schematic of a conventional process. FIGS. 2(a) and 3(a) are sectional views of the reticles used, FIGS. 2(b) and 3(b) represent the amplitude of light transmitting through the reticles, FIGS. 2(c) and 3(c) illustrate the amplitude of light on the wafers, and FIGS. 2(d) and 3(d) show the intensity of light on the wafers. Reference numeral 1 indicates substrate, 2 a light-blocking layer, 3 a phase shifter, and 4 incident light.
In the conventional arrangement, the substrate 1 made up of glass or other material is simply provided with the light-blocking layer 2 for the purpose of defining a light transmitting portion according to a given pattern, as shown in FIG. 3(a). In the phase shift lithographic arrangement, however, the phase shifter 3 that is made up of a light transmitting film is provided on a part of neighboring light transmitting regions on the reticle for the purpose of inducing phase reversal (with a phase difference of 180.degree.), as shown in FIG. 2(a). In the conventional process, therefore, the amplitude of light on the reticle is in the same phase, as shown in FIG. 3(b), and so is the amplitude of light on the wafer, as shown in FIG. 3(c). In other words, it is impossible to separate the resist patterns on the wafer from each other. In the case of phase lithography, in contrast, the light passing through the phase shifter is in opposite phase between neighboring patterns, as can be seen from FIG. 2(b), so that the intensity of light can be zero at the boundary of the adjacent patterns, and this enables the neighboring patterns to be distinctly separated from each other, as can be seen from FIG. 2(d). Thus, phase lithography makes it possible to separate patterns which, until now, could not be separated from each other, resulting in an improvement in resolution.
Various phase shift masks having such a phase shift layer have so far been studied in the art, and each has merits and disadvantages. Here conventional fabrication photosteps will be explained with reference to the Shibuya-Levenson type that is found to have the greatest effect on improving the critical resolution.
One typical example of the conventional process of fabricating phase shift reticles will now be explained with reference to FIGS. 4(a)-4(m) is a sectional schematic of the photosteps of fabricating a phase shift reticle of the type wherein a phase shifter layer lies over a metal thin film structure for light blocking purposes, this type having some relation to claim 2 of the appended claims. In FIGS. 4(a)-4(m), reference numeral 11 indicates a substrate, 12 a metal thin film layer for blocking light, 13 a resist layer, 14 ionizing radiation, 15 a resist pattern, 16 etching gas plasma, 17 a light-blocking metal thin film pattern, 18 oxygen plasma, 19 a phase shifter layer, 20 a resist layer, 21 ionizing radiation, 22 a resist pattern, 23 gas plasma for etching the shifter layer, 24 a phase shifter pattern, and 25 oxygen plasma. As shown in FIG. 4(a), the optically polished substrate 11 is first provided with the metal thin film layer 12, and an ionizing radiation resist such as one made up of chloromethylated polystyrene is then uniformly coated thereon by spin coating or other suitable means. Subsequent drying-by-heating treatment gives the resist layer 13 about 0.1 to 0.2 .mu.m thickness. The drying-by-heating treatment, although varying with the type of resist and the type of equipment used, is done at 80.degree. to 180.degree. C. for 20 to 60 minutes in the case of using an oven, and for about 1 to 30 minutes in the case of using a hot plate.
Then, as shown in FIG. 4(b), a pattern is drawn on the resist layer 13 by ionizing radiation 14 in a manner, using an electron beam exposure device or other equipment. Subsequently, the resist layer is developed with a developer containing an organic solvent such as ethyl cellosolve or ester as a main component, and rinsed with alcohol, etc., to form a resist pattern such as one shown at 15 in FIG. 4(c).
Subsequently and if required, heating and descumming are done to remove unnecessary residue and scum remaining on the edge or other portions of the resist pattern 15. Following this, as shown in FIG. 4(d), the unexposed or unpatterned region of the metal thin film layer 12 is etched dry with the etching gas plasma 16 to form the light-blocking metal thin film pattern 17. It is noted that the step of etching the metal thin film layer may be done wet in place of dry.
After this, as shown in FIG. 4(e), the resist pattern 15 is incinerated out by the oxygen plasma 18 to complete a photomask having the light-blocking layer 17 formed of the metal thin film layer, as shown in FIG. 4(f). It is noted that this step may be carried out by using solvent releasing in place of the incineration treatment using oxygen plasma.
Subsequently, this photomask is inspected and, if required, repaired , followed by washing. Thereafter, the phase shifter layer 19 is formed on the light-blocking layer, as shown in FIG. 4(g). As shown in FIG. 4(h), the ionizing radiation resist 20 is formed on the phase shifter layer 19 in the same manner as mentioned above, and, as shown in FIG. 4(i), alignment drawing for the pattern 17 is carried out with respect to the resist layer 20, using an electron beam exposure device or other equipment. The resist layer 20 is thereafter developed and rinsed to obtain a given resist pattern such as the one shown at 22 in FIG. 4(j).
If required, heating and descumming are done, after which, as shown in FIG. 4(k), the unexposed or unpatterned region of the phase shifter layer 19 is etched dry with the etching gas plasma 23 to form the phase shifter pattern 24. It is noted that the phase shifter pattern 24 may be formed by using wet etching instead of using dry etching with the etching gas plasma 23.
Then, the remaining resist is incinerated out by the oxygen plasma 25, as shown in FIG. 4(l).
Through the above-mentioned steps, a phase shift reticle having a phase shifter layer such as one shown at 24 in FIG. 4(m) can be completed.
In the above-mentioned conventional process for fabricating a phase shift reticle, the phase shifter is made by forming a resist layer on a phase shifter layer, conducting alignment for pattern drawing with the use of an electron beam exposure device or a laser lithography device, developing the resist layer, and dry etching the phase shifter layer, using the resist pattern as a mask. However, a problem with this process is that if a part of the chromium nitride oxide, etc., forming the light-blocking film pattern is left unmasked or unprotected, the surface of the phase shifter layer is then damaged. This problem occurs even when the phase shifter layer lies below the light-blocking metal thin film layer--which has some relation to claim 1 of the appended claims.
In particular, this defect is found in oxidized, nitrided and carbonized chromium films that are used for making the surface layers of light-blocking structures mainly used with photomasks less reflective to light. This problem may be solved, if the surface of a light-blocking layer remains protected during drying etching, as already proposed by the present applicant in JP-A-3-47850.
In carrying this out, however, two additional photolithographic steps are needed for pattern drawing and back exposure prior to dry etching the phase shifter layer, resulting in an increase in the number of the steps involved. The increase in the number of the steps possibly gives rise to another grave problem such as a throughput drop and a defect rate increase.